Explanation
\displaystyle y=\dfrac{F/A}{\dfrac{\Delta l}{l}}
\displaystyle \Delta l=\frac{Fl}{Ay}
\displaystyle \Delta l=\frac{Mgl}{Ay}
\displaystyle \Delta l=\frac{1\times 10\times 1.1}{1\times 10^{-6}\times 1.1\times 10^{11}}=1\times 10^{-4}=0.1\:mm
\displaystyle Y=\frac{F/A}{\frac{\Delta l}{l}} \displaystyle F=\frac{YA\Delta l}{l}
\displaystyle F=Y.\pi r^{2}\frac{\Delta l}{l}
\displaystyle F=0.9\times 10^{11}\times 3.14\times \left ( \frac{0.6}{2}\times 10^{-3} \right )^{2}\times \frac{0.2}{100}
\displaystyle F=0.9\times 10^{11}\times 3.14\times \times 0.09\times 10^{-6}\times 2\times 10^{-3}
\displaystyle F=51\:N
\displaystyle Y=\frac{F/A}{\Delta l/l} ; Y & \displaystyle \frac{\Delta l}{l}=constant
\displaystyle F\propto A\propto r^{2} \displaystyle \frac{F_{2}}{F_{1}}=\left ( \frac{r_{2}}{r_{1}} \right )^{2}=\left ( \frac{4r}{r} \right )^{2}=16
F_{2}=16\times 10^{3}\:N
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